1. Field of the Invention
Aspects of the present invention generally relate to an image capture apparatus having a variable translucency mirror.
2. Description of the Related Art
Digital single-lens reflex cameras (DSLR cameras) have become increasingly popular among photographers as a high-quality camera option, due to their many advantageous characteristics. In a typical DSLR camera, light received by the camera lens is directed via a mirror to an optical viewfinder, allowing the user to compose and even focus an image for photographing using the main optics of the camera. Many high-end DSLR cameras also provide higher quality and larger image sensors as compared to compact cameras, and may also provide several other professional grade-type options, such as the ability to select from among a variety of interchangeable lenses.
Yet another advantage of certain conventional DSLR cameras is their ability to provide relatively quick focus of an image for capturing. One of the techniques used in DSLR cameras to provide rapid focusing is phase-detection autofocus. Phase-detection autofocus generally involves receiving light rays from different parts of the lens, and comparing the images from the light rays to determine a phase difference therebetween. An advantage of this technique is that it provides for the determination of not only whether an image is out of focus, but also the direction and the extent to which the image is out of focus. Focus correction measures can thus be rapidly implemented using this information, such as by adjusting the focal length of the lens in a direction and amount calculated for correction of the phase difference. In certain DSLR cameras, phase-detection autofocus is facilitated by providing a mirror in the camera that reflects incoming light rays simultaneously towards both a viewfinder and phase-detection autofocus sensors. When a picture is taken, the mirror is “flipped up” or otherwise moved out of the path of the incoming light, thereby allowing a sufficient amount of light to fall on the image sensor. Examples of phase-detection autofocus techniques are described in U.S. Pat. No. 5,589,909 to Kusaka and U.S. Pat. No. 4,952,966 to Ishida et al, both of which are incorporated by reference herein in their entireties.
Recently, newer DSLR cameras incorporate a “Live View” feature, in which a display image is generated from light received by the image sensor, and this image is displayed to a user of the camera, generally on an LCD or other display screen built into the camera body. The “Live View” mode can be advantageous in that the displayed image provides a preview of the exposure of the photograph to the user. However, DSLR cameras having such “Live View” modes have the disadvantage in that phase-detection autofocus cannot be performed at the same time. This is because any mirrors or other objects that would block or otherwise re-direct light away from the image sensor are typically moved out of the light path during the “Live View” mode, such that the amount of light received by the image sensor is sufficient for the image display. This includes any mirrors employed in the camera to re-direct light towards phase-detection sensors. Accordingly, phase-detection autofocus correction is not operable in the “Live View” mode, because the phase-detection sensors do not receive the light necessary to perform phase-detection autofocus.
Instead, cameras having such “Live View” modes often utilize an alternative method for autofocus correction, generally referred to as contrast detection autofocus. In contrast detection autofocus, the light received by the image sensor itself can be used to determine a position of a lens that provides the greatest contrast, signifying that the image is in focus. However, a problem with the contrast detection autofocus is that it can be slow when compared to phase-detection autofocus techniques. This is because, while the contrast detection autofocus technique is capable of determining whether an image out of focus, the technique typically does not provide information on the direction or extent to which the image is out of focus. Thus, trial and error methods are used in arriving at the proper focus parameters, such as by moving the lens back and forth to evaluate the contrast at different lens positions. The contrast detection autofocus method may also be especially slow in DSLR cameras, because such cameras can have a depth of field that is shallower than compact (non-DSLR) cameras, and DSLR cameras may also have a relatively larger lens that does not allow for changing of the focal length as quickly. Furthermore, the data processing involved in contrast detection autofocus is typically more exhaustive than that required by phase-detection autofocus, making the processing for the contrast detection autofocus slow in comparison.
In an attempt to resolve such issues, a number of different solutions have been proposed. As an example of such a proposed solution, the FUJIFILM F300 EXR compact camera implements a phase-detection autofocus technique that uses micro-lenses and half-pixel masks on the image sensor. The micro-lenses and half-pixel masks help to separate and distinguish light coming from the left and right sides of the lens, thereby allowing for the phase-detection autofocus using the light received by the masked pixels.
However, a problem with implementing this technique is that the micro-lenses and half-pixel masks can reduce by half the amount of light detected by these pixels, thereby reducing the signal-to-noise ratio (SNR). While image processing can be performed to improve the appearance of the resulting image, the noise nonetheless becomes apparent in low-light conditions that typically amplify such noise. Furthermore, the artificial positions of the half-masked pixels can make the noise look artificial when amplified. Such additional noise can be deemed unacceptable to users of high-end cameras and DSLR cameras where high image quality is desired. Also, the micro lens and half-mask technique used in the FUJIFILM 300 EXR is intended for use in compact cameras, and has not been used in DSLR cameras having optical viewfinder capabilities.
Yet another example of a proposed solution is the SONY A55V, which uses a fixed translucent mirror that directs 30% of incident light to a phase-detection sensor, and allows 70% of the light to pass therethrough to an imaging sensor. Since the translucent mirror allows light to hit both the image sensor and the phase-detection sensor, phase-detection autofocus correction to be performed simultaneously with use of the “Live View” mode. Furthermore, since the translucent mirror allows for a majority of the light to pass therethrough to the image sensor, the camera can be operated with the translucent mirror remaining fixed in position while image capture is performed, without requiring the mirror to move out of the path of incident light.
Along these lines, fixed pellicle mirrors (half-silvered mirrors) have also been employed in SLR film cameras as semi-transparent mirrors that split the incident light beam into a beam directed towards the image sensor and a beam directed towards the autofocus and metering sensors.
However, a problem with such fixed translucent and/or semi-transparent mirrors, as in the SONY A55V, is that by directing a part of the light towards the phase-detection sensor, the amount of light that is received by the image sensor for capturing the image is inevitably reduced. Thus, while the SONY A55V may be capable of providing for phase-detection autofocus in a “Live View” mode, it also suffers from a reduction in the total amount of light that reaches the image sensor. This deficit in available light can be significant, especially in low light conditions where photographers typically seek to use as much available light as possible to produce relatively noise-free images. While a user can increase the exposure time to attempt to compensate for this light deficit, lengthy exposure times can cause blurring of the resulting pictures. The SONY A55V also does not provide for the option of using an optical viewfinder, which is highly desired by certain users, and instead only allows for previewing of an image via the “Live View” mode, similar to a compact camera.
One attempt to address the problem of light deficit during image capture involves providing a translucent mirror that it is movable within the camera body, such that during image capture, the translucent mirror moves upwards and out of the light pathway, thereby allowing the image sensor to capture the full amount of available light. This operation can be similar to the “flipping up” of the fully reflective mirrors used in conventional DSLR cameras, as discussed above. An example of such a movable translucent mirror is described in U.S. Patent Application Publication No. 2010/0045853 to Murashima, which is herein incorporated by reference in its entirety.
Nonetheless, a problem that remains with cameras having translucent mirrors is that they do not use standard optical viewfinders, and instead substitute with electronic viewfinders (EVFs). This is because the translucent mirror allows only a fraction of the light to be directed away from the image sensor. If a standard optical viewfinder were provided in place of the electronic viewfinder, the image viewable therethrough would be excessively dark and unusable, due the deficiency in the amount of light directed towards the viewfinder. Also, since the autofocus sensors are in the path of any light re-directed from the image sensor, their presence would also affect the amount of light received by an optical viewfinder.
Furthermore, EVFs are typically not preferred over standard optical viewfinders by many photographers, because of the problems they can pose. For example, EVFs tend to display an excessive amount of noise in low-light conditions, and the resolution of EVFs also typically do not match that of standard optical viewfinders. Also, as EVFs often use field sequential LCDs to increase the viewfinder resolution, R, G and B signals may be displayed one after another rather than simultaneously in a single pixel, leading the EVFs to display rainbow artifacts, which can be a substantial drawback in their use.
Accordingly, there remains a need for an image capture apparatus and method that allows a user to preview an image by either using an optical viewfinder or by displaying the image on a display (“Live View”), while providing good autofocus correction. There is further a need for an image capture apparatus and method that provides for phase-detection autofocus correction both in a case where the image is previewed using an optical viewfinder, as well as in a case where the image is previewed by displaying the image on a display (“Live View”). There is further a need for an image capture apparatus and method that provides phase-detection autofocus of an image without excessively reducing the amount of light available for image preview and/or image capture.